Some internal combustion engines include engine control systems that deactivate cylinders under low load situations. For example, an eight cylinder engine can be operated using four cylinders to improve fuel economy by reducing pumping losses. This process is generally referred to as displacement on demand or DOD. Operation using all of the engine cylinders is referred to as an activated mode. A deactivated mode refers to operation using less than all of the cylinders of the engine (one or more cylinders not active).
In the deactivated mode, there are fewer cylinders operating. As a result, there is less drive torque available to drive the vehicle driveline and accessories (e.g., alternator, coolant pump, A/C compressor). Engine efficiency, however, is increased as a result of decreased fuel consumption (i.e., no fuel supplied to the deactivated cylinders) and decreased engine pumping. Because the deactivated cylinders do not take in air, overall engine pumping losses are reduced. During typical engine operation in the deactivated mode, the engine is switched to the activated mode when the torque demand is greater than a threshold (e.g., approximately 95%) of the maximum torque available in the deactivated mode.
During the course of normal driving, there are many operating conditions just above the threshold of the deactivated mode torque limit. As a result, there are multiple occurrences of switching to the less fuel efficient activated mode. Once in the activated mode, hysteresis often delays transition back into the deactivated mode. These conditions result in missed opportunities to reduce fuel consumption.